"Off the shelf"....usually means stock.. that's why.
Stock springs
stock cam lift
stock rpms
Even with adjustable rocker arms...you want the correct length push rod to keep that adjuster in the range of 2 threads above the lock nut and 2 to 4 threads below. So deck the block to square or zero deck for the pistons, deck the heads, change the valve job...new valves...
Now see if you can run an off the shelf push rod in there . Now some like comp list in increments of .050.. and could be seen as off the shelf 'though they are cutting them per order' ..but if you need one inbetween .050.. its custom cut. The cup come different sizes...according to the ball end dimension. That can be tight when matched..unlike the stock style with cereal bowls for cups..so not much oil is held. The adjusters arent pissing like the shaft is, the leak is tiny at the adjuster..its leaking oil. Psi loss..? not really. Measure it for us if you think can. Block the adjuster feet somehow and then see if you see an oil pressure game on your gauge. Good luck lol